The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to compressor oil management for HVAC systems.
HVAC systems, such as chillers, often use a flooded or falling film evaporator to facilitate a thermal energy exchange between a refrigerant in the evaporator and a medium flowing in a number of evaporator tubes positioned in the evaporator. The compressor in such systems requires lubrication, typically via oil, to remain operational. As such, a portion of the oil used to lubricate the compressor intermingles with the flow of refrigerant through the compressor and finds its way into the refrigerant flow to the evaporator. When the system is at full load, the refrigerant in the evaporator is continuously contaminated with between about 1% and 5% oil. At partial load, vapor velocity in the evaporator is not sufficient to carry oil from the evaporator to the suction line, so oil accumulates in the evaporator. It is desired to remove the oil from the evaporator for at least two reasons. First, the oil is needed to lubricate the compressor, so it is desired to return the oil to the compressor to replenish a supply thereat. Without doing so, the oil will eventually be depleted from the compressor oil sump. Second, the oil in the evaporator degrades the performance of the system, in particular, the evaporator.
Chillers and other HVAC systems often include an oil management system in a effort to ensure a continuous supply of oil to the compressor. Such an oil management system typically includes an ejector, essentially a pump, which is run continuously to remove refrigerant-rich oil from the evaporator. The ejector uses compressor discharge gas as its working fluid to draw the oil-rich refrigerant from the evaporator and transport it, together with the discharge gas, back to the compressor. This operation, in a typical system, results in about 1% to 2% additional energy consumption by the HVAC system. Further, the typical oil management system leaves the evaporator refrigerant charge continuously contaminated with about 1.5% to 3% oil. This continual contamination reduces overall heat transfer performance of the evaporator by about 3% to 10%. Additionally, in HVAC systems utilizing low pressure refrigerants, the oil contamination causes a reduction in refrigerant vapor pressure resulting in up to an additional about 1% in HVAC system energy consumption.